As our industry evolves, product development and implementation of more energy-efficient solutions increase building performance. Insulation materials and assemblies, including roof insulation, are part of that ongoing process. Better understanding of the dynamics of low-slope roof assemblies helps in the design of better-performing roofs, from a thermal point of view as well as other considerations such as resistance to wind uplift and moisture management.
The recommended practice in the roofing industry is to use two layers of insulation and stagger the joints. By doing so, if gaps open between insulation boards, the joints in the first layer will not be positioned directly over those in the second layer. Consequently, the thermal efficiency of the roof assembly is considered to be less compromised when gaps appear, because there is always an insulation layer that covers a gap. However, that does not prevent insulation from shrinking, so there could still be some appearance of gaps of various widths in those materials.
All materials have a coefficient of thermal expansion that dictates how much they will expand and contract when temperature changes. Materials can also be tested for dimensional stability, which will indicate the level of permanent deformation they will exhibit after exposure to extreme temperatures. If poor dimensional stability of insulation boards causes them to shrink, gaps will appear, which can lead to the creation of wrinkles in the waterproofing membranes.
Wrinkling is not something anyone wants to see in their roof membranes given it can be a sign of undue stress and may lead to premature failures, such as opening of overlaps, abrasion of the raised surface, and ponding of water.
What is the impact of insulation gaps in the thermal efficiency of roof assemblies? A laboratory-based research study recently set out to find the answers. In order to define parameters for the testing, conditions by which insulation materials shrink needed to be considered.
The impact of the type of insulation, membrane, and protection board—as well as their fastening methods—was investigated by exposing roofing system mockups to temperature cycles in a climate chamber. These tests could replicate roof system creasing and ridging as observed in the field.